The present invention relates to electronic circuits and more specifically, to methods and circuitry for optimizing the operating characteristics of driver designs which function efficiently and independently of loading conditions.
Driver circuits or digital peripheral drivers may be described as interface devices which are frequently employed to switch high current, high voltage loads in response to standard digital logic input signals. Examples of such loads may include relays, solenoids, lamps, or other peripheral circuit elements.
In any driver circuit design, one of the most important design considerations is the conservation of drive current to an output transistor. This requires a careful analysis of numerous worst case conditions including process variations which affect a change in transistor beta or resistor values, as well as the effects of temperature and voltage variations on transistor betas, resistor values, and base to emitter voltages (Vbe). Once these conditions are well understood, a circuit designer may elect to implement a circuit which demonstrates a drive current having a value of the worst case drive current plus a 50% overdrive.
While the above approach to designing driver circuits guarantees operation within the range of anticipated variations, the resulting circuits generate excessive drive currents under optimum operating conditions as well as undesirable power dissipation. Moreover, many power drivers in the past have been designed to have current ratings which are two to four times the actual load which results in even further excessive waste of current. Finally, driver circuits are typically designed to operate with a particular load. Any change of loading condition to such designs may result in undesirable operating characteristics. Namely, replacing the "designed load" with one that requires less drive current is at best inefficient, while substituting a load that requires greater drive current may render the driver insufficient. In the later situation, the driver may in fact come out of the saturated state and dissipate excessive power. This in turn may cause a thermally generated loss of functionality if the device is thermally protected and a catastrophic device failure if it is not.
Accordingly, a need has arisen for a high efficiency, high gain driver circuit that is suitable for power applications. In particular, a need has arisen for a driver circuit that optimizes the utilization of drive current for a variety of loading conditions. Such a driver circuit should ideally be suitable for implementation in integrated circuit form and capable of fulfilling the need for such a design in a broad variety of applications including high power automotive and portable equipment operating environments.
It is a primary object of this invention to provide an improved current driver circuit which provides unlimited drive current that is self adjusted to the load requirements and which employs over current limiting. Another object is to provide a current driver circuit which may be used for both high and low side driver applications. Yet another object is to provide a current driver circuit which dissipates minimal power and reduced standby current. Finally, it is a further object to provide a current driver circuit that requires no input current to turn off.